The invention relates in general to the measurement of fluid velocity and in particular to the measurement of water velocity in shallow streams.
The Price pygmy meter is one of the primary tools used by the U.S. Geological Survey to accurately measure water velocity. The original Price pygmy meter uses a contact switch that is difficult to properly adjust so that it works well at low and high velocities. The contact switch also has a very noisy signal that makes accurate counting of the contact closure difficult when using digital electronic devices. A non-contact switch is needed, but the small size of the meter has made a solution difficult.
Prior switches used in the Price pygmy meter include the contact or “wiper” switch, the Swoffer optic switch and the JBS Hall effect switch. The contact or “wiper” switch is sometimes referred to as the “cat whisker” switch and is the original switch used since the inception of the Price pygmy meter. However, the cat whisker switch is difficult to adjust so that it works properly over a wide range of velocities. Adjustments to the cat whisker switch affect the rated performance of the meter, especially at low velocities, thereby reducing the meter's accuracy.
Optic switches work well until water intrudes and refracts the emitted light thereby preventing the switch from closing. A watertight submersible meter is difficult and expensive to build. In addition, the optic switch will not work with a simple two wire connection. Hall effect switches open and close based on opposite ends of the magnetic pole. The Hall effect switch is open half the time and closed half the time, thereby adversely affecting the counting accuracy at low velocities. The Hall effect switch is a solid state device that requires continuous current to operate, making a simple two wire connection more complicated. A currently used Hall effect switch uses a dedicated three wire cable that extends from the meter up to the digital counter, thereby exposing several feet of wire to water velocities.